The present invention relates generally to systems and methods for performing noninvasive surgical procedures using focused ultrasound, and more particularly to systems and methods for creating longer necrosed volumes using a focused ultrasound transducer array.
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors and obviate the need for invasive surgery. In order to generate sufficient energy to ablate or necrose tissue, piezoelectric transducers have been used. Such transducers are placed external to the patient but in close proximity to the tissue to be ablated and are driven by electric signals to produce ultrasonic energy. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused at a focal zone corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of sonications is used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., has been suggested for this purpose. This spherical cap transducer array includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or sectors, creating a sector-vortex array. The transducer elements are all simultaneously driven by radio frequency (RF) electrical signals at a single frequency offset in phase and amplitude. In particular, the phase and amplitude of the respective drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired focal zone and provide a desired energy level in the target tissue region.
To increase the size of the necrosed region, the amplitude of the respective drive signals may be increased, thereby directing more ultrasonic energy at the focal zone. This generally increases the size of the tissue region that is necrosed at the focal zone by the sonication, and consequently may reduce the number of sonications needed to treat an entire tissue structure, such as a tumor. Increasing the amplitude, however, also increases the amount of energy that passes through the tissue on either side of the focal zone. This may cause undesired pain to the patient, heating, and/or necrosis of tissue outside of the target region, particularly in the xe2x80x9cnear field,xe2x80x9d i.e., the region between the transducer and the focal zone.
As an alternative, to increase the necrosed volume per sonication, a spherical cap transducer array has been suggested that is divided circumferentially into a plurality of xe2x80x9csectors,xe2x80x9d such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al. The phase of the respective drive signals to each of the sectors may be controlled to create a desired size and shape for the focal zone. For example, if each of the sectors are driven with respective drive signals that are in phase with one another (xe2x80x9cmode 0xe2x80x9d), the ultrasonic energy may be focused substantially at a relatively narrow focal zone, similar to the concentric ring transducer array described above.
Alternatively, the sectors may be driven with respective drive signals that are in a predetermined phase relationship with one another (referred to as xe2x80x9cout of phasexe2x80x9d or xe2x80x9cmode nxe2x80x9d). This results in a focal zone that generally has a ring shape, creating a wider focus that causes necrosis of a larger tissue region within a focal plane intersecting the focal zone. Achieving adequate necrosis within this larger focal zone, however, requires increasing the total sonication energy (increasing sonication power and/or duration), which may also increase the heating of tissue outside the target tissue region, because more energy may flow through the funnels on either side of the focal zone. This may require additional cooling time between sonications to prevent build-up of heat, particularly within the tissue in the near field.
To increase the size of the necrosed volume in a direction substantially perpendicular to the focal plane, a method known as xe2x80x9capodizationxe2x80x9d has been suggested. Apodization involves activating only an inner set of rings within a concentric ring array during a sonication, i.e., shutting down one or more of the outer rings for the entire duration of the sonication. This effectively raises the xe2x80x9cf-numberxe2x80x9d of the transducer array (f-number is the ratio of the radius of curvature to the size of the xe2x80x9caperturexe2x80x9d or diameter of the transducer array), producing a narrower funnel through which the ultrasonic energy passes between the transducer and the focal zone. When the same total amount of energy is focused at the focal zone using apodization, a longer necrosed volume results. However, the narrower funnel may also increase the risk of excessive heating of the tissue on either side of the focal zone, particularly in the near field.
Accordingly, it would be desirable to provide systems and methods for treating a tissue region using ultrasound energy that may increase the necrosed volume, without substantially increasing the risk of heating outside the focal zone.
The present invention is directed to systems and methods for performing a therapeutic procedure using focused ultrasound that increases the necrosed volume, without substantially increasing heating of tissue outside of the focal zone.
In a preferred embodiment, a focused ultrasound system includes a transducer array including a plurality of transducer elements. Drive circuitry is coupled to the transducer elements, the drive circuitry configured for providing respective drive signals to each of the transducer elements. A controller is provided for alternatively directing ultrasonic energy from sets of the transducer elements towards a patient being treated. Preferably, the controller is coupled to the drive circuitry, the controller configured for controlling the drive circuitry to alternatively provide each set of transducer elements with respective drive signals.
The controller may also be configured for controlling a phase component of respective drive signals provided to the transducer elements to provide a predetermined size and shape of a focal zone created by the transducer elements and/or to focus the transducer elements in each of the sets substantially at a desired focal location. The controller may include a selector for alternatively coupling one of the sets of transducer elements to the drive circuitry, whereby the drive circuitry may provide respective drive signals to the coupled set of transducer elements, and/or the controller may include a microprocessor for electronically controlling the drive circuitry.
In an exemplary embodiment, the transducer array may be a concentric ring array including a plurality of sets of concentric annular transducer elements or rings. Each of the concentric rings may be divided circumferentially into a plurality of curved elements or xe2x80x9csectors.xe2x80x9d Alternatively, other shapes, arrangements, or geometries of transducer elements may be provided that may be divided into sets.
Each of the sets of transducer elements may be alternatively driven with a set of respective drive signals for a predetermined duration during a sonication, while substantially continuously focusing ultrasonic energy produced by the transducer elements of each set at a desired focal zone. Consequently, the resulting necrosed tissue region at the focal zone may be substantially increased in volume, while distributing the energy along separate funnels that substantially reduce the risk of undesired heating and/or necrosis outside the focal zone.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.